My 4 year old daughter loves to dig. This year we had an archeological dig at the beach, but any sandbox will do. We hand painted treasure (unfinished, not perfect throw away pottery from a nearby potter), large stones were painted into gold nuggets. We found lots of miscellaneous items at the dollar store (dinosaurs, beanie fish, plastic gold fish, sponges, etc).  The treasure was buried and each of the kids got a shovel and bucket.  All of the treasure was put into a museum for display during the party. At the end of the party, each child got a treasure chest (plastic pencil box)and taking turns chose their favorite items until there was nothing left in the museum.  This was a very easy idea to implement and the kids had a great time.